The present invention relates to blowout preventers and more particularly to variable bore packers for a ram-type blowout preventer which can be used for sealing different diameter tubular members extending through the blowout preventer and still more particularly to variable bore packers used in high pressure and high temperature wells.
Blowout preventers maintain control of downhole pressure in wells during drilling, and ram-type blowout preventers are used to close and seal around a string of pipe extending into the well to contain the pressure within the well. Variable bore packers have been designed for ram-type blowout preventers to close and seal around tubular members having different diameters within a limited range of sizes. Variable bore packers are designed to adjust their sealing engagement to the particular size of tubular member passing through the ram-type blowout preventer. Various types of prior art variable bore packers have been utilized.
U.S. Pat. No. 4,229,012 discloses a variable bore packer for a ram-type blowout preventer in which irising inserts, operated like a camera shutter, are embedded in the resilient packer and each include an upper plate, a lower plate and a rib connected between the upper and lower plates. Each of the plates is generally triangular in shape and designed to rotate as it moves inwardly with the resilient packer annulus so that the resilient material is supported when in sealing engagement with the exterior of a tubular member extending through the preventer. Also, a linkage structure is provided to allow the desired movement of the packer in sealing while maintaining its connection to the ram.
U.S. Pat. No. 5,005,802 discloses a variable bore packer having an upper and lower plate embedded in resilient packer material. A series of upper insert segments are positioned in the packer material below the upper plate and are removable with the packer material as it moves forward during sealing. The insert segments move inward with the packer material in sealing to provide an upper anti-extrusion support for the packer material upon sealing engagement around the exterior of a tubular member extending through the blowout preventer. The insert segments include an inner radius sized to match the outside diameter of the pipe against which it is to seal. The insert segments also include a radial length which is sufficiently long to allow them to move into engagement with a pipe exterior and still provide support for the resilient packer material to avoid its extrusion.
As variable bore packers sealingly engage tubular strings of different sizes, it is important to prevent the extrusion of the resilient packer material between the variable bore packer and the tubular member. Prior art packers continue to be subject to extrusion such that upon closing the variable bore packer around the tubular member, minute gaps continue to exist between the packer and tubular member. Such gaps become an increasing problem as the packer wears and is abraded by its sealing engagement with various tubular members passing through the blowout preventer. At times it is necessary to perform a "stripping" operation to strip the string through the closed rams. This stripping movement can severely wear or abrade the face of the resilient packer material.
The problem of extrusion is enhanced with increased downhole pressure and/or increased temperature. As downhole pressures increase to 15,000 psi, such large downhole pressures exacerbate the problem of extrusion due to the great pressure differential across the packer. Seventy or eighty cycles is a typical life span for ambient temperature packers. In high temperature packers, however, much more wear occurs in one cycle as in an ambient temperature packer. Further, as temperatures increase to high temperatures in the order of 350.degree. F., the viscosity of the resilient packer material decreases causing it to be more fluid and thereby more susceptible to extrusion through the minute gaps between the packer and tubular member.
The variable bore packer of U.S. Pat. No. 4,229,012 does not lend itself to high temperature applications because it does not create a tight seal around the tubular member. The irising inserts cannot conform well to the diameter of the tubular member and leave a plurality of small gaps allowing extrusion by the less viscous packer material.
Various prior art packers have introduced filler material into the elastomer of the resilient packer material. U.S. Pat. No. 4,398,729 discloses a pipe ram with a removable packer insert made from HYTREL, a proprietary DuPont elastomer. U.S. Pat. No. 4,323,256 discloses a pipe ram with a packer insert made of a low friction material. The preferred material is stated as being Teflon with moly and fiberglass. U.S. Pat. No. 4,506,858 discloses a non-variable ram front packer with layers of reinforcing fabric molded into the elastomer to strengthen the elastomer. The fabric is a various combination of polyaramid, nylon and cotton duck. U.S. Pat. No. 4,553,730 discloses molding layers on non-metallic fabric into the top portion of a pipe ram packer to minimize the elastomer extrusion and also offer improved wear resistance during "stripping". Polyester fabric is listed as being a possible material for the non-metallic fabric.
A cross-section of wire has been used in bonnet seals. It is also known to use knitted wire mesh or braided wire in the packer material immediately adjacent the face of the wear plates to limit extrusion of the material. U.S. Pat. No. 4,428,592 also discloses a pipe ram with a packer having wire mesh molded into the packer face to resist wear during "stripping". U.S. Pat. No. 4,219,204 suggests the use of such knitted wire in a seal as an anti-extrusion means. It is also known to embed a canvass fabric in seals, such as mud pump piston seal rings, to provide extended seal life.
Polyester rope has been previously used in static elastomeric seals as an anti-extrusion material. Small diameter polyester rope is used to fill a space or crack through which the rope will not pass. For example, polyester rope has been used in wellhead seals.
It is also common industry practice to pre-shrink polyester or nylon rope prior to molding it into a rubber part. The pre-shrinking of the rope prevents it from later shrinking in the part when exposed to the high temperatures of the mold. Although polyester and nylon rope have previously been used for static seals, it is not known to use such rope for seals that change shape to conform to any of several sealing diameters.